1. Field of the Invention
The present invention relates to a vehicle navigation control system for navigating a vehicle along a path so as to assist a driving operation of a driver.
2. Description of the Related Art
In recent years, the steering operations of drivers and operable stabilities of vehicles are becoming important matters from a safety point of view. Technologies that allow drivers to be free from tedious driving operations and vehicles to automatically run are being developed.
A navigation control is a basic technology for allowing vehicles to run on a designated path safely and correctly.
Generally, except for undulations, paths for vehicles consist of curved lines and straight lines. To cause a vehicle to follow a curved path, a steering angle corresponding to the curvature of the curved path should be given position by position.
The following vehicle navigation models for navigating the vehicle corresponding to the curved path are known.
1) Programmed steering model
A predetermined steering pattern is selected corresponding to steering characteristics of the vehicle and pattern recognition of a forward course. After the vehicle enters the curved path, the steering angle of the vehicle is controlled corresponding to the selected steering pattern.
2) Forward error compensation model with primary prediction.
As shown in FIG. 7A, steering angle .delta. is given by multiplying error width d of extended line 0 of vehicle 1 against a target path at a predetermined forward location by a predetermined proportional constant (gain) k (namely, .delta.=k.times.d).
3) Forward error compensation model with secondary prediction.
A forward location of the vehicle is predicted with present location, direction, and running conditions. The predicted location is compared with a target path. The vehicle is navigated so that the difference between the predicted location and the target path becomes zero. As shown in FIG. 7B, the error width .epsilon. for a predetermined forward observing distance is integrated and the resultant value is multiplied by the proportional constant k. The result is the steering angle .delta. (namely, .delta.=k .SIGMA..epsilon.).
These simulations are described in for example Automobile Technologies, "Automobiles and Ergonomics Topic 2", Vol. 25, No. 10, 1971, pp 1058-1064.
However, in the programmed steering model, since many of shapes should be considered for paths, optimum navigation characteristics cannot be obtained. Thus, the number of programming steps becomes huge. In addition, as the shape of a path largely varies, the response delay becomes large.
In the forward error compensation model with primary prediction, navigation control characteristics for nearly straight paths are satisfactory. However, if the vehicle runs on the curved path with a small curvature, a gain (proportional constant) corresponding to the curvature would be required so as to precisely navigate the vehicle on the path.
In the forward error compensation model with the secondary prediction, even if the curvature of the path varied, if a predetermined gain (proportional constant) was given, satisfactory navigation characteristics could be obtained. However, since error width .epsilon. was integrated, if the vehicle runs on the path with a large curvature (namely, on an almost straight path), the speed of the vehicle increases. Thus, the navigation control becomes unstable. In addition, if the predetermined forward distance is far from the vehicle on the path where the curvature largely varies position by position (namely, on an S-shaped path), the curvature of the path on which the vehicle runs would largely differ from the curvature of the target path. Thus, the vehicle would be out of the path.
The present invention is made from the above-mentioned point of view.